ATI is a company, it is not a technology. They have produced a variety of video chipsets over the years, with different capabilities and differing support requirements. You will have to determine what video chipset is embedded in your proposed equipment.

The ati(4) and radeon(4) drivers list the specific chipsets that are supported for ATI video technologies. Note that these links to the web-based man pages default to OpenBSD-current. You may select the most recent release, 4.3, once on the page. On November 1, you will be able to select 4.4, which is releasing on that day.

The reason I point out the different man pages for -current, 4.3, and 4.4 is that there have been recent changes to radeon chipset support, including hardware acceleration.

Video is not the only compatibility concern -- since notebook and laptop computers have differing (and sometimes not-quite-standard) ACPI implementation, you should probably try to select a notebook already tested and discussed in www.openbsd.org/i386-laptop.html

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If you are checking out notebook computers at a store where you have the opportunity to try before buying, you might consider bringing along a LiveCD or LiveDVD version of the OS for testing. You can see how the OS behaves with any hardware before committing to it.

At the moment, i386 and amd64 versions of 4.3 are available at the link in my .sig, below. On November 1, 4.4 versions will be available, with the possibility of sparc64 and macppc architecture ISOs also eventually appearing.